The present invention relates to a sealing arrangement for fuel cells including at least one composite formed of two cell separator plates with a deformable membrane electrode assembly placed therebetween. The deformable membrane electrode assembly is composed of two porous, gas-permeable plates or layers and an ion-exchange membrane placed therebetween, the lateral surfaces of the membrane electrode assembly being set back with respect to the lateral surfaces of the cell separator plates to leave a sealing gap. The present invention relates as well to an elastic sealing element enclosing the composite in the manner of a peripheral sealing strip.
Fuel cells are electrochemical energy converters and are well-known. They produce electric energy by oxidizing a fuel. In the simplest case, they are composed of planar, electrically conductive electrodes which are gas-permeable and separated from each other by an ion-conducting membrane. The reaction media are supplied via distribution plates having integrated gas- or liquid-conveying channels. These distribution systems have to be sealed both from each other and from the outside. To produce an electric voltage or an electric current of a technically usable magnitude, usually a plurality of large-surface, thin plates or layers are arranged above each other in the form of a stack and the individual cells are interconnected in series or parallel. The electric energy produced by the converter is tapped at electrically conductive electrodes of the stack.
In the simplest case, such an electrochemical fuel cell is composed of two electrodes, designed and referred to in literature as a planar “gas diffusion layer”, hereinafter abbreviated as GDL, between which is located an ion-conducting layer, each electrode having an adjacent gas space in which in each case one reaction medium is supplied via distribution channels. Seals between the individual cell elements prevent the reaction medium from escaping.
In certain fuel cells, the ion-conducting membrane is a polymer. The present invention relates to the sealing of such a polymer electrolyte membrane fuel cell, hereinafter referred to in short as “PEM cells”. This type of chemical fuel cells is increasingly gaining importance as a future energy source for the propulsion of motor vehicles. The requirements for this application include as favorable a mass/power ratio as possible and a sealing of the distribution systems which remains reliable over several years.
In polymer electrolyte membrane fuel cells, the two porous, gas-permeable electrodes and the very thin proton-conducting polymer electrolyte membrane placed therebetween are usually combined into a so-called “membrane electrode assembly”, hereinafter abbreviated as MEA. When arranged in the stack, these assemblies are separated by so-called “cell separator plates”. The latter are provided with the above-mentioned distribution structures for the reaction gases in the surface. The stack is terminated with end plates on each of the end faces and held together by tie bolts, pressing the layers together. Often, nonmetals, such as graphite, but also metals, such as high-grade steel or titanium, are used for the electron-conducting cell separator plates. A suitable electrode material for the anode or cathode is plastically deformable and electrically conductive material such as graphite films or non-woven fabric materials. The electrode surface contacting the polymer electrolyte membrane is coated with a catalyst, for example, a platinum material. Cell separator plates within the stack are in electrical contact with the anode of a cell of the stack via one of their surfaces while their opposite surface is in contact with the cathode of another, adjacent cell. According to this function, these cell separator plates within the stack are also referred to as so-called “bipolar plates”, hereinafter referred to in short as “BPP”. Apart from their function of conducting the electric current in the stack, they also have the function of separating the reaction gases.
For a PEM fuel cell, usually, hydrogen is used as the reaction gas and oxygen or air are typically used as the oxidizing agent. Hydrogen is supplied to the anode chamber formed by the distribution structure on the anode while the oxygen or air is supplied to the cathode chamber. Via the gas-permeable electrodes, the reactants reach the proton-conductive ion-exchange membrane through the catalyst layer. Cations forming at the catalyst layer of the anode migrate through the ion-exchange membrane and react with the oxidizing agent supplied at the cathode side to produce, on one hand, water as a reaction product and, one the other hand, electric and thermal energy. The electric energy can be supplied to a load via a an external electric circuit while the thermal energy in the stack has to be dissipated through suitable cooling channels between the cell separator plates.
High demands are placed on the seals between the individual cell elements. PEM fuel cells which are intended to supply energy to a motor vehicle are exposed to rough environmental conditions. The seal has to withstand heavy vibrations, humidity fluctuations and variations in temperature. Leaks can occur due to different material expansion.
To seal the gas spaces and the fluid collection channels, German Patent Application No. 197 13 250 proposes a gas- and liquid-tight adhesive composite of the membrane electrode assembly with the adjacent cell separator plates in the manner of a peripheral seal. The adhesive composite material is achieved by an adhesive agent which interconnects the cell elements in a marginal region, forming a gas-tight seal. The lateral surfaces of the membrane electrode assembly are set back with respect to the lateral surfaces of the cell separator plates, thus forming a sealing gap which is filled by the adhesive composite material and protects the polymer electrolyte membrane from desiccation. Several such modules can be connected by coating the end faces of the stack with adhesive composite material. The handling of the adhesive agent, which needs to be accurately applied in the marginal region, is a disadvantage during production. Another disadvantage is the undetachable connection in a stack of fuel cells as a result of which the whole stack must be discarded when one cell is defective.